There's a Combobox option tk_setPalette,
but the name implies a canvas background, I don't know this to be fact, just an assumption.
If this is true, you can probably use pmw to create a custom window that will
allow individual item colors, but out of the box, I think you're stuck with a single color.
Howvere, nothing is to stop you from coloring the text of each item.
Output:
| tk_setPalette(self, *args, **kw)
| Set a new color scheme for all widget elements.
|
| A single color as argument will cause that all colors of Tk
| widget elements are derived from this.
| Alternatively several keyword parameters and its associated
| colors can be given. The following keywords are valid:
| activeBackground, foreground, selectColor,
| activeForeground, highlightBackground, selectBackground,
| background, highlightColor, selectForeground,
| disabledForeground, insertBackground, troughColor.